Conventionally, in a fixing process of an image forming apparatus, as a departure between an actual temperature and a target temperature of a nip part heated by a lamp is suppressed, a duty control for repeating an ON state and an OFF state of the lamp which is a heating source in a short cycle is executed. However, in the conventional duty control, as a switching frequency of the ON state and the OFF state of the lamp increases, the lifetime of the lamp undesirably decreases.